There are several methods as focus detection schemes for cameras. The device disclosed in Japanese Patent Laid-Open No. 58-24105 is designed to perform focus detection based on a pupil division scheme using a two-dimensional light-receiving sensor having a microlens formed at each pixel on the sensor.
FIG. 2 is a view for explaining the principle of the method of performing focus detection based on the pupil division scheme proposed in the above reference.
An image sensor 10 is placed on a prospective imaging plane of a photographing lens 5. One pixel of the image sensor 10 is constituted by two photoelectric conversion elements 13α and 13β. The photoelectric conversion elements 13α and 13β are designed to receive light beams transmitted through different positions on the pupil of the photographing lens 5 through a microlens 11 formed on the photographing lens side of each photoelectric conversion element.
In this case, the photoelectric conversion element 13α mainly receives a light beam transmitted through the upper side of the photographing lens 5 in FIG. 2, whereas the photoelectric conversion element 13β mainly receives a light beam transmitted through the lower side of the pupil of the photographing lens 5 in FIG. 2. In focus detection, outputs from the respective photoelectric conversion elements are read, and an image signal based on light beams transmitted through different pupil positions on the photographing lens is generated from outputs from a plurality of pixels.
The present applicant disclosed a focus detection device capable of performing focus detection based on the pupil division scheme in Japanese Patent Laid-Open No. 2001-124984. The focus detection device in this reference has a pair of photoelectric conversion elements provided for each of two-dimensionally arranged microlens arrays, and divides the pupil by projecting the photoelectric conversion elements on the pupil of a photographing optical system through the microlenses. The focus state of the photographing lens is detected on the basis of the phase difference between the first and second image signals (to be referred to as the A image signal and B image signal hereinafter, respectively) obtained from outputs from the photoelectric conversion element array corresponding to two light beams passing through different portions of the pupil of the photographing optical system. In this case, the phase difference indicates the relative positional relationship between the A and B image signals.
A focus detection device is also disclosed in Japanese Patent Publication No. 7-62731, which realizes accurate focus detection by switching the F-numbers of a focus detection optical system upon detection of an unbalanced state of the A and b image signals obtained from outputs from a photoelectric conversion element array. This focus detection device has two focus detection units with different F-numbers. When the coincidence between the A and B image signals is high, focus detection is performed by the focus detection unit with the smaller F-number that allows the reception of many light beams. When the coincidence between the A and B image signals is low, focus detection is performed by using the focus detection unit with the larger F-number that minimizes vignetting of a focus detection light beam.
An image capturing device designed to perform gain control on a pixel signal on the basis of lens information and a position on an image sensing plane is also disclosed in Japanese Patent Laid-Open No. 2000-324505. Letting L be the distance from the center of the image sensing plane to a pixel, a gain control amount for the pixel at an arbitrary position is calculated by a polynomial for L. In this case, the coefficients of the polynomial are determined on the basis of a data table stored in correspondence with lens information including the lens exit pupil position.
In the focus detection device disclosed in Japanese Patent Publication No. 7-62731, in order to detect an unbalanced state of the A and B image signals obtained from outputs from photoelectric conversion cells, correlation computation processing must be performed once. For this reason, a computer with a high processing speed is mounted in the focus detection device to result in an increase in cost. Alternatively, a computer with a low processing speed is mounted in the device to result in a failure to realize high-speed focus detection. Furthermore, depending on a focus detection area, focus detection cannot be performed by using light beams with a small F-number.
In the image capturing device disclosed in Japanese Patent Laid-Open No. 2000-324505, letting L be the distance from the image sensing plane center to a pixel, a gain control amount on the pixel at an arbitrary position is calculated by a polynomial for L. Assume that this technique is applied to the focus detection device disclosed in Japanese Patent Laid-Open No. 2001-124984, i.e., gain control is performed for each of the first and second image signals according to a polynomial for L. In this case, if the gain control amount to be obtained is expressed by a polynomial for L, the degree of the polynomial increases. As a consequence, the calculation amount increases to result in an increase in computation load. For this reason, a computer with a high processing speed is mounted in the focus detection device to result in an increase in cost. Alternatively, a computer with a low processing speed is mounted in the device to result in a failure to realize high-speed focus detection.